The invention concerns a crankshaft bearing shell for a combustion engine of an automotive vehicle and made from a steel/aluminum composite material having a steel support layer and a plated aluminum sliding layer with tin and copper portions in the aluminum sliding layer, optionally comprising an intermediate pure aluminum layer.
Due to the increasing power of engines, crankshaft-bearing shells for combustion engines of automotive vehicles must meet increasingly higher requirements concerning fatigue strength and achieve high hardness and strength values while maintaining excellent tribological properties.
Sliding layer shells of this type have been known for a long time having an aluminum sliding layer consisting of an aluminum alloy of material designation AlSn20Cu composed of                17.5 to 22.5% per weight tin        0.7 to 1.3% per weight copper and        remainder aluminumwith optional further alloy components of up to 0.7% silicon, 0.1% magnesium, 0.1% nickel, 0.2% titanium, 0.7% iron, 0.7% manganese and impurities of a total of less than 0.5% (all above amounts are % per weight). The fatigue strength of such crankshaft bearing shells during operation is however limited to a maximum load of 35 to 45 N/mm2. This fatigue strength represents the load due to ignition and inertial forces acting on the projected bearing surface.        
Crankshaft bearing shells comprising the above-mentioned aluminum sliding layer can be used without additional galvanic sliding layer (overlay-free) due to the high tin content, and have excellent tribological properties. The relatively high tin content produces thick, elongated, linear tin deposits during casting and rolling of the material at which cracks may form, and therefore only achieves the average fatigue strength region mentioned above.
EP 0 704 545 A1 discloses a steel/aluminum composite material having a tin content in the medium range of between 14 and 16% per weight and a copper content of 1.7 to 2.3% per weight in the aluminum sliding layer. The low tin content combined with thermal treatment in the region of 200 to 220° C. improved the fatigue strength and load capacity thereby facilitating use of this composite material in connecting rod bearings for the large connecting rod eye. The seizure resistance required for use as a crankshaft bearing shell was not obtained since the tin content was reduced relative to that of the AlSn20Cu alloy.
The high requirements for seizure resistance in crankshaft bearing shells result from unavoidable misalignment and shape deviations during production of the crankshaft despite the most demanding honing and finishing processes for the crankshaft and the bearing block and their effects under extreme loads and at high sliding velocities of more than 10 m/s which occur in modern combustion engines.
DT 1 521 196 discloses use of an aluminum alloy with 10 to 35, preferably 18 to 22% tin, 0.5 to 2% copper, and suggests thermal treatment at 230° to 425° C. of the bimetal strips produced through rolling of the aluminum alloy onto a steel backing. Very large tin deposits characterize a steel/aluminum composite material, which is treated at this temperature. It has a very low fatigue strength and low hardness and would not be suited for production of crankshaft bearing shells for modern combustion engines. Nor does the very broad range of copper content of between 0.5 and 2% help to further define the sliding bearing material.
DD 50319 describes a method for producing sliding bodies from aluminum alloys with up to 30% tin and 3% copper. The document mentions a steel mandrel for the production of the bore of bushings, and the undesired effect of gravitational segregation, which suggests a continuous casting method for monometallic parts. The fact that up to 30% of tin and up to 30% of copper are added, wherein the copper should generally increase hardness, does not provide clear definition of the aluminum sliding bearing layer. This document also mentions that a tin portion of up to 18% was suggested in prior art to thereby obtain competitive results with regard to bearing load.
Based on these findings, it is the underlying purpose of the present invention to produce a crankshaft bearing shell of the above-mentioned type which can withstand a load during operation of the engine of considerably more than 55 N/mm2 at a high sliding speed of more than 10 m/s while nevertheless maintaining good sliding properties comparable to those of the above-mentioned crankshaft bearing shell having a sliding layer of AlSn20Cu.